It is required that a conductive material used for forming a conductive circuit on a circuit board such as a printed wiring board or an interlayer connecting between layers be chemically stable and have a low electric resistance. On the other hand, the circuit board such as the printed wiring board where an electronic component is mounted is required to have a structure where conductive circuits are formed with high densities in a multi-layer manner. Accordingly, in recent years, a printed wiring board where IVHs (Inner Via Holes) or blind via holes are formed with high density has been used. The IVHs or the blind via holes are formed by embedding the conductive materials in through holes or via holes so that upper and lower layers are connected to each other. With this structure, it is possible to achieve high density mounting of the electronic components.
As a method forming a conductive circuit on such a circuit board, a subtractive method has been used. In the subtractive method, a metal film as a conductive circuit is formed on an entire surface of the printed wiring board and then unnecessary metal film is etched by using a photolithography technique so that the conductive circuit is formed on the circuit board. In addition, as another method of forming the conductive circuit on the circuit board, the following method has been suggested for reducing the manufacturing cost. That is, a conductive paste film including particles of a conductive material such as silver (Ag), copper (Cu) or carbon (C) and a binder dissolved by a solvent is printed on the surface of the circuit board by a screen printing method so that a circuit is formed.
In addition, the following method is also suggested. That is, the IVH or the blind via hole is filled with a conductive paste and a curing process is applied so that a conductive circuit connects the interlayer of the circuit board.
Furthermore, a conductive material including plural particles having an electrically conductive coating, where some of the particles are fused to each other by the electrically conductive coating, has been suggested. See Japanese Laid-open Patent Application Publication No. 8-227613.
In addition, the following method has been suggested. That is, a conductive filler, where a film of tin (Sn) having a thickness of 1 μm is formed on surfaces of copper (Cu) particles by an electroless plating method, and a designated agent are mixed so that a conductive paste is formed. The conductive paste is printed by a screen printing method so that a conductive circuit and via plugs are formed. See Japanese Laid-open Patent Application Publication No. 2006-19306.
Furthermore, a conductive metal paste used for a terminal electrode of a monolithic ceramic electronic component has been suggested. See Japanese Laid-open Patent Application Publication No. 2-46603. In this conductive metal paste, an alloy formed of two kinds of metallic elements whose melting point is lower than the melting points of the elements, and an intermetallic compound formed of the two kinds of the metallic elements having high melting points, are used. In this suggestion, a compound metal powder where zinc (Zn) is coated on a surface of a copper (Cu) powder is used as a terminal electrode of the monolithic ceramic electronic component. By sintering at 500° C. through 600° C., diffusion progresses between the zinc (Zn) and the copper (Cu) so that part of the powder becomes brass and a high density sintered body is obtained. In addition, the remaining copper (Cu) is surrounded by the brass so that oxidization of an electrode surface of the copper (Cu) terminal is prevented.